1. Field of the invention
The present invention concerns an improved device for controlling movement of a mobile member such as an optical element of a stellar interferometry system or, more generally, an interferometry device such as a FOURIER transform spectrometer.
Its object is to stabilize dynamically a mobile member during a programmed movement so as to eliminate the effect of vibrations.
2. Description of the Prior Art
Programmed vibration-free displacement of this kind is needed, for example, to control an optical delay line within a coherent recombination type stellar interferometer. In this case, where the distance (called the base) between the telescopes is in the order of 100 meters, equalizing the optical paths, despite the rotation of the Earth, requires displacement of the reflector of the optical delay line over a distance of several meters, at a speed of several tens of millimeters per second, with an average positional accuracy of a few microns and with an instantaneous accuracy (during a setting time of a few hundredths of a second) of a few nanometers (amplitude of the vibrations): the constraints are therefore more severe in respect of the dynamic error than in respect of the static error.
Such displacement characteristics are also necessary if the optical paths are equalized by moving the telescopes themselves.
A doctoral thesis by L. KOECHLIN submitted to the University of Nice on 15 Nov. 1985 under the title "Exploitation astrophysique de l'interferometre adeux telescopes du CERGA" ("Astrophysical application of the CERGA two-telescope interferometer") there is described an optical bench for recombining the interferometer beams serving as a delay line.
This table moves on accurately machined rails. It is driven by a torque motor with position and speed control. The transmission system uses a precision lead screw and nut coupling.
This type of displacement device has the disadvantage of producing vibration, especially at high speeds (a few millimeters per second), such vibration originating from the guidance system and also from the transmission. This vibration compromises the accuracy of the fringe contrast measurements.
An air cushion guidance system represents an improvement, but flow instabilities often generate non-negligible high-frequency vibrations.
The 1986 MIT thesis by R.W. HOLM entitled "A High Speed High-Resolution Optical Delay Line for Stellar Interferometry Applications" discloses a device for displacing a "cat's eye" reflector in a delay line comprising a slave carriage moving on rails and comprising a chassis mobile on flexible pivots to which the cat's eye is fixed.
The secondary mirror of the cat's eye is carried by a piezo-electric actuator. The position of the mobile chassis relative to the carriage is controlled by a displacement detector and a loudspeaker voice coil type linear actuator. Finally, the carriage is moved by a stepper motor.
The optical path introduced by the delay line is measured by a laser interferometer. The error signal is injected into the control loop of the piezo-electric actuator. Saturation of this actuator is avoided by the action of the linear actuator and saturation of the linear actuator is avoided by the action of the stepper motor (hence the above use of the term "slave"). There are therefore three control loops nested one within the other.
This displacement device makes it possible to achieve vibration-free displacement of the cat's eye similar to friction-free displacement. It offers good performance because the error signal corresponds directly to the magnitude to be controlled, that is to say to the optical path.
On the other hand, it is ill suited to high speeds because the laser measurement system would then have to have a resolution of a few nanometers while the ratio of the maximum speed of variation of the optical path of the carriage to the resolution is limited by the electronic circuitry for reasons of feasibility.
Also, the piezo-electric actuator cannot apply sufficient force for this device to be applicable to the movement of large masses such as telescopes.
Finally, this device is costly because of the presence of the three control loops and because of the need to have a high-performance laser measurement system which can only function correctly in a vacuum.
A device of this type is described in the publication: P. CONNES 1975 Applied Optics, Vol 14 No 9, p 2067-2084.
An object of the present invention is to describe a device for programmed displacement of a carriage carrying, through a flexible linkage, a payload which must be moved without vibration over great distances and at high speed, as in the delay lines of a stellar interferometer, and having none of the disadvantages mentioned above.